1. Field of the Invention
The present invention relates to a mica tape that is wound a plurality of times around the outer circumferential surface of a conductor of an electrical rotating machine coil so as to form an electric insulating layer, to an electrical rotating machine coil prepared by winding the mica tape around the electrical conductor so as to form a good insulating layer having a good heat conductivity, and to an electrical rotating machine using the electrical rotating machine coil.
2. Description of the Related Art
It is known to the art that a coil is used as a stator winding or a rotor winding in an electrical rotating machine such as a dynamo or an electric motor. The coil is prepared by winding a plurality of times a mica tape around an electrical conductor formed of, for example, a flat element wire having a rectangular cross section so as to impart electrical insulating properties to the coil, followed by heating under pressure the mica tape together with an adhesive resin so as to form an integral structure of an insulating layer.
FIG. 7 is a cross-sectional view exemplifying the construction of a stator coil of an electrical rotating machine including an insulating layer formed of the conventional mica tape.
As shown in FIG. 7, a slot 53 is formed in a stator core 51 prepared by laminating a plurality of electromagnetic steel plates one upon the other.
A stator coil 52 formed of an element wire having a rectangular gross section is housed in the stator core 51. The stator coil 52 comprises a conductor portion 52a and an insulating layer 52b formed around the conductor portion 52a. In general, the stator coil 52 has a double layer structure including an upper coil and a lower coil. In this case, however, the stator coil 52 alone is shown as the lower coil in the drawing for the sake of brevity. It should be noted that under the lower coil 52 is arranged a spacer 54 in a space between the lower coil 52 and the bottom portion of the slot 53.
In some cases, it is possible for a high voltage and a large current to be applied to the conductor portion 52a of the coil 52 depending on the specification of the electrical rotating machine. Such being the situation, an insulating layer 52b is formed around the conductor portion 52a in order to avoid an accident such as the ground short-circuit caused by the applied high voltage. For forming the insulating layer 52b, it was customary in the past to wind a plurality of times a mica tape 61 having a cross-sectional structure as shown in FIG. 8 around the conductor portion 52a, followed by heating the mica tape under pressure so as to form an integral structure of the insulating layer 52b excellent in the electrical insulating properties.
As shown in FIG. 8, the mica tape 61 comprises a mica layer 63 including a plurality of scale-like mica foils 62 that are laminated one upon the other so as to form a layer-like structure and a glass cloth 64 laminated below the mica layer 63 for reinforcing the mica layer 63. The glass cloth 64 is prepared by weaving glass fibers 65 in vertical and lateral directions. Mica is widely used as a material excellent in electrical insulating properties. However, since the bonding strength among the scale-like mica foils is weak, it is difficult to use a tape prepared by simply bonding the mica foils for the fastening purpose.
Such being the situation, the glass cloth 64 prepared by vertically and laterally weaving the glass fibers 65 is included for the reinforcing purpose in the mica tape 61 so as to make it possible to use the mica tape 61 to be wound around the conductor portion 52a with a high tension.
On the other hand, if a large current flows within the conductor portion 52a of the coil 52, Joule heat is generated within the conductor portion 52a because of the Joule loss. Therefore, it is necessary to remove the Joule heat generation efficiently. The Joule heat generation can be removed by the cooling with, for example, a hydrogen gas or the air. In any case, it is necessary to remove the heat from the conductor portion 52a via the insulating layer 52b. 
Since the heat conductivity of the insulating layer 52b greatly affects the entire cooling capacity, it was customary to study in the past how to improve the heat conductivity of the insulating layer 52b. To be more specific, it was proposed in the past to add an inorganic filler (not shown) as a material having a high heat conductivity to the glass cloth 64 included in the mica tape 61.
A mica tape containing aluminum oxide as an inorganic filler having a high heat conductivity is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2003-9446.
Inorganic fillers excellent in the heat conductivity also include aluminum nitride and silicon nitride in addition to the aluminum oxide noted above.
However, any of the inorganic fillers excellent in the heat conductivity exemplified above has a high hardness. For example, aluminum oxide has a Mohs hardness of 12.
As described previously with reference to FIG. 8, the glass cloth 64 prepared by weaving the glass fibers 65 is used for reinforcing the mica tape 61. It should be noted that glass has a Mohs harness of about 7. In the case of using a mica tape prepared by adding aluminum oxide having a Mohs hardness of 12 as a filler, it was possible for the aluminum oxide to damage the glass cloth 64 particularly at the corner portion of the conductor when the mica tape is wound about the conductive wire having a rectangular cross section, with the result that the glass cloth 64 is broken, leading possibly to the breakage of the mica tape.
Particularly, in order to form the insulating layer 52b as strong as possible, the mica tape is wound a plurality of times about the element wire having a rectangular cross section. In winding the mica tape, it is necessary to increase the tension of the mica tape as much as possible so as to permit the mica tape to be brought into contact more strongly with the element wire. With increase in the tension noted above, the compressive strength applied within the mica tape is increased, with the result that the planar pressure applied between the glass cloth and the inorganic filler is also increased. It follows that the mica tape tends to be broken easily so as to make it impossible to increase the tension as much as required. As a result, a gap is generated between the formed insulating layer and the surfaces of the wire element having a rectangular cross section. Such being the situation, the heat dissipation and the insulating properties were not sufficiently satisfactory.